1. Field of the Invention
This invention relates to a pole face design for superconducting magnets of the type that are C-shaped. Such structures of this type, generally, employ pole faces which homogenize the magnet field within the imaging volume.
2. Description of the Related Art
The present superconducting magnetic resonance imaging (MRI) magnets require cryogens to operate, either liquid helium or liquid helium and liquid nitrogen. Liquid helium is used in superconducting magnets not only for cooling but also to stabilize the magnet windings against motion induced instabilities. Cryogens evaporate and are lost during magnet operation and, therefore, require periodic cryogen delivery service and cryogen addition, with the attendant cryogen safety hazards. Furthermore, the use of cryogens complicate the cryostat construction since the cryogen containment vessels must be built in accordance with pressure vessel codes to withstand pressure surges as a result of magnet quenches or loss of vacuum in the vessel surrounding the cryogen containment vessel. The heavy cryostat containment vessel requires complicated supports and shields to position the cryostat containment vessel in the vacuum vessel and yet minimize heat conduction and radiation to the containment vessel from the ambient temperatures surrounding the vacuum vessel.
Helium leaks into the vacuum vessel surrounding the cryostat containment vessel are a common cause of failure in most superconducting magnets. Helium gas leaking into the vacuum vessel provides high heat conduction from the ambient temperature surrounding the vacuum vessel to the cryogen containment vessel containing the superconducting coils.
Cryogen costs, specifically helium, are increasing and there is a limited supply which is economically recoverable. In many parts of the world helium is not available, therefore MRI systems based on niobium titanium (NbTi) superconductors cannot be operated. Therefore, a more advantageous superconducting magnet, then, would be presented if such amounts of the cryogen could be substantially reduced or eliminated.
Exemplary of such prior art magnets which do not employ cryogens and which also allow patients to be imaged while either standing up or lying down is commonly assigned U.S. Pat. No. 4,924,198 ('198) to E. T. Laskaris, entitled "Superconducting Magnetic Resonance Magnet Without Cryogens". However, while it is advantageous to be able to image patients while they are in a variety of positions, the magnet must be constructed such that the patients are capable of entering the magnet and being imaged quickly such that the patients should not experience any claustrophobia while being imaged. In particular, the '198 patent employs supports which provide a gap between the coil pairs. These supports are needed in order to provide adequate structural strength for the coil pairs. However, the imaging process could be quickened especially when imaging is being done on a patient who is standing up, if the patient could simply walk into an open area between the coil pairs instead of having to walk around the supports. Also, just prior to or while the patient is being imaged, if the patient did not see the supports, possibly any anxiety or claustrophobia the patient may have, would be eliminated. Consequently, a still further advantageous superconducting magnet, then, would be presented if such amounts of imaging time and possible patient anxiety or claustrophobia could be reduced while avoiding the use of cryogens.
Exemplary of such prior art superconducting magnets which do not employ cryogens and which reduce patient anxiety or claustrophobia is commonly assigned U.S. Pat. No. 5,153,546 ('546), entitled "Open MRI Magnet" to E. T. Laskaris. While the '546 patent has met with a degree of commercial success, it is made up, of necessity, of a large amount of superconducting tape. Typically, over 50,000 feet of superconducting tape are required in the '546 patent. Presently, superconducting tape costs over twenty cents per foot. Therefore, the cost of the superconducting tape alone in the '546 patent can make the '546 patent quite expensive. Also, it would be desirable to be able to shim the magnet so as to homogenize the magnetic field within the imaging volume. Therefore, an even still further advantageous superconducting magnet, then, would be presented if such amounts of superconducting tape were reduced while eliminating claustrophobic effects and cryogens and homogenizing the magnetic field within the imaging volume.
It is apparent from the above that there exists a need in the art for a superconducting magnet which is cryogen free, and which is capable of eliminating the claustrophobic effects associated with enclosed superconducting magnets, which at the same time uses only a small amount of superconducting tape while being capable of homogenizing the magnetic field within the imaging volume. It is a purpose of this invention to fill this and other needs in the art in a manner more apparent to the skilled artist and once given the following disclosure.